Tiny GE 7077 UHF planar triode, measuring 12x12mm (the penny is 20mm
across). 6.3V/240mA heater. This little valve is designed for use in grounded-grid
amplifiers. It was used as a Class B final amplifier in the Pioneer III
space probe.

Markings are 67-39 and 68-04.

Heater connections are the buttons at the bottom, the grid connection
is the ring, and the anode is the plate on top.

.

Thanks to John Kaesehagen for this info, and Dave for the extra additions
to the collection.